Adding a knee bone for better definition to Rigify metarig
You can add knee bones to a Rigify rig but you need to do this to the generated rig, not to the meta rig.
(The meta rig is used by Rigify to define the location, shape, and size of the generated rig.
The generated rig will eventually be used for posing. It consists of numerous bones, such as deform bones (that deform the mesh and start with DEF- prefix in their names), control bones (for IK/FK), and mechanism bones, etc. All these bones can be found on the different bone layers.)
But Rigify has a custom property named &quot;Rubber Tweak&quot; for the shins (knees) and elbows that can give those areas more volume.
For comparison:

If you want to add an extra knee bone you can find the deform bones on bone layer 30. Add a knee bone between the thigh and the shin bone as described in the video:

Clear Parent (Alt+P) to separate them
select the ends of the bone, F to add the knee bone
rename it to DEF-knee.L
parent the shin bone to the new knee bone and check the connected checkbox

Additionally, you need to add 2 bone constraints to the new knee bone (switch to Pose mode, see  for details).

Rubber tweak option (right leg), extra knee bone (left leg)
Note: There is a bug (Blender 2.91), that makes Rigify not generate all needed layers. The bug happens now and then when you have edited the bones and press the Generate Rig button. Check if all layers are there before you proceed. Are Arm.R (IK) and Arm.L (FK) present? If they are missing have a look here:
Rigify-&gt;Generate Rig doesn&#39;t create all rig layers in the properties ui